Among the most commonly used equipment for protecting humans from exposure to toxic materials such as volatile organic compounds are gas masks or respirators and related or similar devices. Toxic gases are encountered in various work environments such as paint booths and laboratories, and chemical warfare agents may be encountered in military or civilian contexts. These gas masks, respirators and related or similar devices have filters that allow the user to breathe air through them, but act as barriers to the toxic materials. The filter typically contains activated carbon or other high surface area materials that rapidly adsorb the toxic gas and prevent it from proceeding along the filter together with the breathable air.
There are two basic ways by which the toxic gas interacts with the adsorbing material. In the first mode, the gas is simply adsorbed reversibly and in the second mode the gas reacts chemically and irreversibly with the adsorbing material or with materials distributed on its surface. In both cases the toxic material is prevented from proceeding along the filter and thus from being breathed by the user. An example of reversible adsorption is that of hydrocarbons or halogenated hydrocarbons on activated carbon. An example of an irreversible reaction is that of hydrocyanic acid with metallic components deposited on the surface of activated carbon.
Every barrier used in gas masks has a finite useful life. Its surface gets saturated and reactive ingredients get consumed by the process of filtering the toxic material. Once consumed, the filtering media ceases to be a barrier and permits the toxic material to move through and poison the breather. This makes the filter ineffective and thus necessitates replacing it or taking other means to protect the user. Since in many cases the user cannot tell that the filter has became ineffective, there is a need to have means to detect that the filter was consumed and to indicate to the user and that he is facing incipient danger.
There are two basic devices in relation to the life of gas masks filters. The first type is called End of Service Life Indicators (ESLI), which indicate that the filter was consumed and cannot be used anymore as a filter. The second is called a Residual Life Indicator (RLI), which indicates how much of the filter was used (or how much is still available for use).
An example of a respirator with an ESLI is described in U.S. Pat. No. 4,155,358. This respirator is a disposable valveless chemical cartridge respirator that removes vinyl chloride monomer from the breathed air and is equipped with an end of service life indicator. The end of service indicator changes its color when the cartridge has been fully utilized. The indicator comprises specially prepared activated alumina granules coated first from a 1% solution of KMnO4, which is then reduced to what is believed to be MnO2, and coated from a 0.55% solution of potassium permanganate. The indicator is viewed through the side wall of the cartridge all along the edge nearest the entrance to the cartridge. The initial color is purple and upon exposure to vinyl chloride, the potassium permanganate is reduced to manganese dioxide such that there is a slow, continuous color change from the purple to brown (the color of manganese dioxide). This design has two key problems. First, it requires a major modification of the casing of conventional gas masks filters and of their manufacturing procedure and machinery, and, second, it indicates only that the cartridge life is beginning to or has expired but it does not permit any estimation or indication of how much life is left in the cartridge.
U.S. Pat. No. 4,146,887 discloses another type of ESLI that consists of two key components: a material that can react exothermally with the gas to be filtered, and a temperature sensor. These two components are placed in the filter or near the face piece cavity of the respirator. Once the polluting (target) gas reaches the absorbent, it reacts with the adsorbent and releases heat. The temperature sensor senses the increase in the temperature of the adsorbent and triggers an alarm when the respirator cartridge has reached the end of its service life. Again, this indicator indicates only that the filter has lost or is about to lose its usefulness. Further, it requires major modification to filters and to filter manufacturing procedures, as well as the provision of power sources to sense the temperature increase and activate the alarm.
Several technologies have been disclosed in which strips are inserted within protective clothing to indicate by color change or other means that gas has permeated therethrough. These devices are not true indicators of the ESL or RSL, but rather of exposure of the person within the protective clothing. Examples of such technologies include U.S. Pat. No. 4,205,043 and U.S. Pat. No. 5,376,554.
U.S. Pat. No. 6,701,864 teaches that the residual life of a filter may be estimated by measuring the size of a stain of colored material placed within the filter and viewed from the outside. This RSLI is useful for specific gases such as hydrocarbons, but its utility is limited since hydrocarbons are adsorbed reversibly and tend to redistribute themselves within the filter. Moreover, the filter design and manufacturing procedure have to be modified to incorporate viewing windows. Since such modifications are rather expensive and are highly undesirable, they limit the commercial utilization of this device.
An object of the present invention is to provide a residual life indicator which, through visual indication such as a color change, will indicate that the useful life of the filter is about to end. For example, the indicator may be set to indicate when only 10% of the filter is left available for use.
Another objective of the present invention is to provide an ESLI that can be used with almost all conventional filters with only a very minimal change in their design.
Another objective of the present invention is to provide a highly versatile RSLI that can be easily be changed to meet the specific safety objectives of the user.
Another objective of the present invention is to provide an RSLI that indicates several levels of utilization of the filter to permit proper or orderly replacement.
Another objective of the present invention is to provide an RSLI or and ESLI that is extremely simple and easy to use even by untrained personnel.
Yet another objective of the present invention is to provide an RSLI or ESLI that does not require any power sources, computers or any other external or instrumental means to enable the user to determine the status and utility of his filter, before any exposure has occurred.
Other objects, features and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.